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CELL BIOLOGY AND SIGNAL TRANSDUCTION

BIN2 Functions Redundantly with Other Arabidopsis GSK3-Like Kinases to Regulate Brassinosteroid Signaling^1,[W],[OA]

Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109–1048

GLYCOGEN SYNTHASE KINASE3 (GSK3) is a highly conserved serine/threonine kinase involved in a variety of developmental signaling processes. The Arabidopsis (Arabidopsis thaliana) genome encodes 10 GSK3-like kinases that are clustered into four groups. Forward genetic screens have so far uncovered eight mutants, all of which carry gain-of-function mutations in BRASSINOSTEROID-INSENSITIVE2 (BIN2), one of the three members in group II. Genetic and biochemical studies have implicated a negative regulatory role for BIN2 in brassinosteroid (BR) signaling. Here, we report the identification of eight ethyl methanesulfonate-mutagenized loss-of-function bin2 alleles and one T-DNA insertional mutation each for BIN2 and its two closest homologs, BIN2-Like1 and BIN2-Like2. Our genetic, biochemical, and physiological assays revealed that despite functional redundancy, BIN2 plays a dominant role among the three group II members in regulating BR signaling. Surprisingly, the bin2bil1bil2 triple T-DNA insertional mutant still responds to BR and accumulates a more phosphorylated form of a BIN2 substrate than the wild-type plant. Using the specific GSK3 inhibitor lithium chloride, we have provided strong circumstantial evidence for the involvement of other Arabidopsis GSK3-like kinases in BR signaling. Interestingly, lithium chloride treatment was able to suppress the gain-of-function bin2-1 mutation but had a much weaker effect on a strong BR receptor mutant, suggesting the presence of a BIN2-independent regulatory step downstream of BR receptor activation.

² Present address: Biotechnology Research Institute, Chinese Academy of Agricultural Sciences, 12 South Zhongguancun Boulevard, Beijing 100081, China.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jianming Li (jian{at}umich.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.138099

^* Corresponding author; e-mail jian{at}umich.edu.

Received March 4, 2009; accepted April 20, 2009; published April 24, 2009.